Apparatus and method for uniformly wetting hard-to-wet powders

ABSTRACT

Apparatus for wetting particulate material comprises a duct 1 that is open at its base 2 and that is provided at its top 3 with a duct inlet 4 defined by inlet walls 5 down which water flows continuously from a weir 6 around the top of the walls. Sprays 19 at or just beneath the duct inlet 4 direct water downwardly so as to wet the particulate material. Preferably the apparatus is used for wetting friable particulate material, in which case the sprays direct water across the path of the downflowing particulate material so as to disintegrate it into individual particles and to wet these. Downflow of air is minimized by providing a cowl 9 over the top of the duct and by feeding the particulate material, generally by a screw feed 13, into a passage that leads from an inlet 15 that is exposed to the atmosphere to an outlet that discharges direct into the duct inlet 4.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a national stage application filed under 35 U.S.C. §371 from International Application No. PCT/GB94/00358, filed Feb. 23,1994, which is a continuation-in-part of U.S. application Ser. No.08/025,360, filed Feb. 26, 1993, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus and method for wetting powderedmaterial, especially polymeric powdered material, so as to promotesubsequent uniform distribution of the wetted material throughout bulkwater.

It is well known that it can be difficult to dissolve water solublepowdered material quickly in water without forming lumps of aggregatedpartially dissolved material, often called fish-eyes.

Numerous mixing devices have been proposed with the aim of convertingdry, substantially friable, particulate material into a uniformdispersion or solution in water.

A successful device is described in U.S. Pat. No. 4,086,663. Thisinvolves feeding particulate solids entrained in a stream of aircentrally into a duct that is open at its lower end and that is providedwith a plurality of sprays around the inlet for the entrained solids.The sprays are arranged to generate a spray of mist particles thatsubstantially fill the duct. The individual polymer particles are wettedsubstantially individually while they pass down through the duct. Theyare collected in bulk water at the bottom of thE duct and, since theyare individually wetted before they contact the water, they can dissolveor swell in the water rapidly and substantially independently of oneanother.

Using this system therefore it is necessary that the polymer fed intothe duct is in the form of individual particles. Particulate solidsstill in a friable or loosely aggregated form must be broken intoindividual particles by entrainment in air before they enter the duct.

Another apparatus that preferably feeds the particulate solids into thedevice while entrained in air is U.S. Pat. No. 4,390,284. The solids maybe fed into the device using a screw feeder. In this, a tangentialstream of water is directed helically around a cone shaped inlet intowhich the entrained stream is directed, so as to wet that cone shapedinlet and prevent build-up of dry material on it. Wetting of theparticulate material is substantially carried out by helically arrangedjets of water in a cylindrical wetting duct. These jets are arranged toinduce a considerable air flow through the cone shaped inlet and wettingduct.

A disadvantage of processes of this type is that they involve a largeamount of air being fed into the top of the duct. Although the majorityof the polymer particles are wetted while travelling through the ductand are collected in the bulk water beneath the duct, a large amount ofair has to escape from the bottom of the duct above the bulk water andthere is a risk that this air will carry away from the apparatus polymerparticles entrained in the air. The risk is particularly significant forpolymer particles that are finer than the majority of the particles forwhich the apparatus is designed. Accordingly the apparatus has atendency to allow polymer fines to be blown away from the foot of theduct and this is undesirable, especially when the polymer powdercontains a significant proportion of fines.

In U.S. Pat. No. 4,531,673 a polymer is fed by a screw feed into the topof a cone shaped duct and so it might be thought that this would reducethe risk of fines being blown out of the bottom of the duct. However theinlet arrangements at the top of the duct are designed to promote a flowof air being induced into and down the duct as a result of thedownwardly directed water sprays. Since the apparatus is designed toinduce this air flow, the problem of fines being blown out of the bottomof the duct still occurs.

In U.S. Pat. No. 4,643,582 the powdered material is fed into an upperconical inlet that leads into a cylindrical duct. A stream of water isdirected tangentially into the conical inlet so as to flood its surface,and water is also directed tangentially into the cylindrical duct.Pressurised air is forced into an annulus around the conical portion andexits from this annulus into the cylindrical duct, thereby inducingsuction into the centre of the conical portion so as to draw the powderinto that. The pressurised air again incurs the risk of fines beingblown out of the bottom. The use of a tangential water supply to floodthe surfaces increases the risk of dry particles being partiallydissolved in this excess water while in contact with one another, thuspromoting the formation of fish-eyes.

It is an object of the invention to provide a simple apparatus andmethod for uniformly wetting water soluble or water swellableparticulate material so as to permit that material to be dissolved oruniformly dispersed in bulk dilution water with minimum formation offish-eyes, whilst maintaining a low level of build-up of solid materialin the apparatus. It is a further object of the invention to achievethis with minimum escape of particulate fines from the apparatus duringuse. It is a preferred object of the invention to provide an apparatusand method for doing the above for water soluble or water swellablefriable particulate material.

SUMMARY OF THE INVENTION

The invention provides apparatus for uniformly wetting water soluble orwater swellable particulate material comprising

a substantially vertical wetting duct that is open at its lower end andthat has at its upper end a duct inlet that is substantially coaxialwith the duct and that is defined by inlet walls, and through which thematerial can be fed to the duct, and

water spray orifices arranged to direct sprays of water downwardly inthe wetting duct to wet the particulate material,

characterised in that the duct inlet has a radius less than the radiusof the duct and the water sprays are positioned substantially around theduct inlet and the apparatus also comprises

weir means extending around the top of the inlet walls,

and means for providing a substantially continuous flow of water overthe weir means and down along substantially the entire exposed surfaceof the inlet walls.

The invention includes the method of using the apparatus. In use theparticulate material is fed by feed means into the duct inlet.

In order to minimise the risk of the particulate material adhering tothe walls of the inlet, these walls are continuously flushed by wateroverflowing the weir means that extend around the top of the walls. As aresult of flushing the walls by water that overflows a weir extendingaround substantially the entire length of the top of the walls it iseasily possible to keep the walls substantially entirely free ofaccumulated particulate material even though the amount of water flowingdown the walls can be rather small. This is in contrast to the knownmethods described above wherein a relatively large amount of water hasto be applied tangentially with the intention of trying to wash theentire wall surface. The most convenient way of providing asubstantially continuous flow of water over the weir means comprisesproviding an annular vessel surrounding the duct inlet and ducting forcontinuous supplies of water to the annular vessel. By adjusting therate of supply of water through that ducting, the rate of flow of waterover the weir and down the inlet walls can be controlled.

The particulate materials for which the invention is useful aregenerally polymeric materials. They can be water-soluble polymers, inwhich event the wetted polymer particles will subsequently be added towater to form solution, or they can be water-swellable butwater-insoluble particles in which event the wetted particles will forma uniform suspension upon addition to water.

The above apparatus and method are useful for uniformly wettingparticulate material introduced into the duct wholly or mainly in theform of individual particles, for example as in U.S. Pat. No. 4,086,663,discussed above.

However, the apparatus and process of the invention are preferably usedfor uniformly wetting water soluble or water swellable friableparticulate material. By referring to friable particulate materials wemean materials which are formed of particles which have tended to clumptogether in loose agglomerates and which remain as agglomerates unlesssteps are taken to separate them.

In an apparatus useful for this purpose the water spray orificespositioned substantially around the duct inlet are preferably arrangedto direct sprays of water downwardly and substantially across the axisof the duct and thereby to disintegrate into substantially independentparticles friable particulate material falling through the duct from theinlet and to wet the substantially independent particles.

In known techniques, the particulate material is often introduced whileentrained in air, in which event initially friable particulate materialis separated into substantially independent particles by the airentrainment. However this necessitates the flow of substantial amountsof air through the duct, and this can result in fines or other materialbeing blown out of the duct. The invention avoids this. In thispreferred aspect of the invention, the particulate material is fed intothe apparatus while still in its friable or loosely agglomerated form.It may be carried into the apparatus by any suitable carrier on whichfriable material may be carried, such as a screw feed or a beltconveyor, resulting inevitably in loose agglomeration of the particlesas they enter the apparatus.

The water spray orifices are positioned substantially around the ductinlet and so can be located at a level that is either slightly below orat the lowermost part of the duct inlet or can be positioned around thesides of the duct inlet or even slightly above the duct inlet.Preferably however they are positioned around the lowermost part of theduct inlet or slightly below it, e.g. 1 to 5 cm below it. Convenientlythe sprays extend from an annular chamber to which pressurised water isapplied and which has an outer diameter substantially the same as thediameter of the duct. Thus the sprays are preferably located around anannulus that has an internal radius that is at least half, andpreferably at least two-thirds, of the radius of the duct.

The sprays serve two purposes. One purpose is to disintegrate thefriable material used in the preferred aspect of the inventionsubstantially into individual particles. Accordingly they must havesufficient power to do this and must be directed across the general lineof travel of the particulate material. The other purpose of the spraysis to provide a reasonably uniform distribution of water dropletsfalling throughout the entire duct so as to maximise the contact of thepolymer particles with water droplets while the particles are fallingthrough the duct. This latter purpose is fulfilled in all forms ofapparatus and all methods according to the invention.

The downward flow of the water sprays will tend to induce a downflow ofair through the duct. In order to control and generally minimise theinduced air flow through the duct the apparatus may include a cowl thatsubstantially entirely encloses the top of the wetting duct. The cowlshould include an aperture to permit the provision of feed means forfeeding the particulate, preferably friable, material to the duct inletfrom outside the cowl. The cowl should also include air inlet means. Theaperture referred to above may also serve as air inlet means. Preferablythe air inlet means are substantially coaxial with the duct. The airinlet means may include a passage which terminates above or inside theduct inlet.

Preferably the feed means discharge direct into this passage. The feedmeans, preferably a conveyer feed means when the material fed is friableparticulate material, may lead from outside the cowl through theaperture into the cowl so as to discharge the particulate material abovethe duct inlet. In such a case the feed means terminate within the cowlbelow the air inlet and above the duct inlet. The air inlet means are insuch a case generally positioned in the top surface of the cowl,separate from the aperture. The passage is also entirely within thecowl.

Preferably however the feed means are entirely outside the cowl anddischarge into the air passage which leads from outside the cowl throughthe aperture to the duct inlet. In the latter case air turbulence andbuild-up of particulate material within the cowl are reduced.

The flow of air from the air inlet will tend to carry the particulatematerial as a stream into the duct inlet. This is particularly usefulwhen the particulate material is not fed entrained in air but is fedinto the apparatus in its friable form.

When the feed means terminate within the cowl, the air inlet means maymerely be an aperture in the top of the cowl. However they mayalternatively be adjustable air inlet means for controlling the amountof air that can be induced to flow into the upper end of the wettingduct. When the feed means terminate outside the cowl at an air passageleading into the cowl the top of the air passage may merely be anaperture or may be adjustable. In some embodiments adjustable air inletmeans may be desirable; in others the overall design may render itunnecessary.

In the preferred aspect of the invention the air passage has an airinlet that is exposed to the atmosphere and a side feed position forreceiving a feed of friable particulate material and leads to an outletthat discharges into the duct inlet. When the material fed is not in thefriable form the material may be fed entrained in air. In such a casethe air inlet means will generally not simply be exposed to theatmosphere but will lead from air entrainment means.

The air passage outlet is preferably of smaller diameter than the ductinlet but can be substantially coextensive with, and positioned above,it if desired. The spacing between the passage outlet and the top of theduct inlet (generally the weir means) is preferably made as small asconveniently possible in order to minimise the amount of air induced bythe sprays to flow in through this spacing, especially when there is nocowl. When the passage outlet is of smaller diameter than the duct inletit preferably extends into the duct inlet. This assists in directing andaccelerating the particulate material into the duct from the passageoutlet.

There may also be provided if desired openings in the upper end of thewall of the wetting duct. These are preferably positioned substantiallyat the same height as the sprays. They may be substantially equallyspaced around the wetting duct wall. Preferably each hole is positionedbetween two adjacent water spray orifices. The holes allow air to bedrawn indirectly into the duct to compensate for the air downflow effectof the water sprays and reduce turbulence and build-up of particulatematerial in the upper end of the duct. Instead of being in the walls ofthe duct, they may, less preferably be between the duct and cowl or evenin the lower part of the cowl.

The preferred particulate materials are water soluble polymers having aparticle size with at least 90%, and usually at least 99%, by weight inthe range 20 to 1000 μm. Because the invention allows minimisation ofthe amount of air flowing through the duct and the risk of fines blowingout of the bottom of the duct, the invention is of particular value whenthe particulate material includes at least 2%, and often at least 5% byweight of material having a particle size in the range 20 to 100 μm,often 20 to 70 μm. The amount of these "fines" can be as high as, forinstance, 30% by weight but is preferably below 15 or 20% by weight.

At least 80%, and usually at least 90%, by weight of the particulatematerial usually has a particle size below 700 μm, frequently below 400μm.

The particulate material can be a natural polymer such as a starch orcellulose but preferably is a synthetic polymer made by polymerisationof water-soluble monomers, optionally with a cross-linking agent if thepolymer is to be swellable and insoluble. The monomers can typically beacrylamide or other non-ionic monomers, sodium acrylate or other anionicmonomers, and dialkylaminoalkyl(meth)-acrylate or -acrylamide acidaddition or quaternary salts or other cationic monomers.

DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the accompanying drawings (which are notto scale) wherein

FIG. 1 is a vertical cross-section through one form of apparatusaccording to the invention.

FIG. 2 is a plan view from below of the assembly of spray nozzles shownin FIG. 1.

FIG. 3 is a perspective view, part broken away, of the cowl.

FIG. 4 is a vertical cross-section through a second form of apparatusaccording to the invention.

FIG. 5 is a perspective view, part broken away, of the cowl of thissecond form of apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One typical apparatus, for wetting friable particulate material, issubstantially cylindrical and is shown in FIGS. 1 to 3 in which the duct1 is shown as having an open lower end 2 and an upper end 3. A ductinlet 4 is positioned coaxial with the duct and is formed of cylindricalinlet wall 5 having an upper edge 6 that serves as a weir from anannular water chamber 7 that is supplied with water through pipes 8. Thefilm of water that can be caused to flow continuously down the walls 5prevents powder sticking to the walls even though some spray dropletsmay enter the inlet from below.

A cowl 9 rests on lugs 10 and makes a reasonably tight fit around thetop of the duct 3 so as to minimise the amount of air that can be drawnin around the base of the walls of the cowl. The cowl is provided withan aperture 11 to permit the insertion of an air passage 20 which has atits top end an air inlet 15. As shown in FIG. 3, the cowl may comprisetwo halves that are hinged at a hinge 25 at the side and that areclipped together by a clip 26 to form the complete cowl.

The duct inlet 4 and the air inlet 15 are substantially coaxial with theduct 1 and are interconnected by the air passage 20 which terminates ata passage outlet 22 inside the duct inlet 4. The passage 20 has asmaller diameter than the duct 4. The apparatus is provided with aconveyer 12 that is typically a screw feed conveyer 13 and that leadsfrom a hopper or other suitable store 14. The conveyor 12 is positionedto discharge loosely agglomerated powder from the hopper 14 into a sidefeed T-piece 21 in the duct 20 at a position between the air inlet 15and duct inlet 4. As a result the powdered material is carried by acontrolled airstream travelling from the inlet 15 to the inlet 4 andwhich is induced by the sprays. The annulus 23 around the chamber 7 isopen, but since the passage 20 discharges direct into the inlet 4 verylittle powder enters this annulus and since the cowl is substantiallyclosed around the passage 20 only a small amount of air flow is induceddown through this annulus.

An annular chamber 17 surrounds the bottom of the inlet duct 4 and isprovided with water supply pipes 18 by which pressurised water can beforced into the annular chamber. Spray jets 19 (shown as 19A, 19B and19C and discussed in more detail below) are arranged around this annulusin such a way that they all point downwardly and inwardly. Their angleto the vertical is generally in the range 5° to 30°, typically 8° to 15°. Although it is convenient for all the spray jets to be similarlyinclined so as to define a cone, as illustrated, it is also possible forthe jets to be mounted at different angles, for instance with some ofthe jets being mounted at an angle of 5° to 20 ° and some of the jetsbeing mounted so as to make a greater angle with the vertical, forinstance 15° to 40°.

In order that the jets break up the downflowing particulate material ispreferred that they operate at a high pressure, typically in the range1.03×10⁵ to 4.14×10⁵ Pa (15 to 60 psi), preferably 1.38×10⁵ to 2.76×10⁵Pa or 4.14×10⁵ Pa (20 to 40 psi or 60 psi), and provide a cone angle ateach jet of from 10° to 100°, preferably 10° to 45°. Preferred jets aresold by Spraying Systems under the trade names Floodjet and Fulljet.Such jets are preferably mounted at centre spacings of 3 to 15 cm, oftenaround 6 cm. Preferably some of the sprays have a cone angle of 30° to90° or 100° and some have an angle of 100° to 140°. The different spraysare preferably substantially uniformly distributed around the duct.Further sprays can be positioned elsewhere in the duct, but this isgenerally undesirable.

Holes 24 are provided in the wall of the duct 1 just below the lugs 10.The holes 24 are equally spaced with each one being placed between apair of spray jets 19. The presence of these holes reduces turbulent airflow in the upper end oft he duct 1 and reduces polymer build up in theapparatus.

The interior surfaces of the duct 1, duct inlet 4, spray jets 19 andside feed T-piece 21 are coated with non-stick material. Thisfacilitates cleaning of the apparatus and contributes to the reductionof polymer build up on the interior surface of the side feed T-piece 21.

On the exterior surface of the side feed T-piece there are providedheating elements. This reduces condensation in this part of theapparatus.

The duct 1 typically has a radius of from 9 to 15 cm, often around 10 to60 cm, generally 10 to 15 cm and a length below the spray orifices offrom 22 to 200 cm, often 22 to 30 cm. The length above the sprayorifices is typically 7 to 15 cm, often 9 to 15 cm. The ratio of thediameter to the length is often from 1:2 to 1:5. The duct inlet 4typically has a radius of from 3 to 5 cm and a length above the sprayorifices of from 1 to 5 cm. The ratio of the diameter to the length isoften from 1:1 to 1:3.

FIG. 2 shows a plan view of the assembly of spray nozzles. For ease ofdisassembly and cleaning of the apparatus the annular chamber 17 may beprovided as an incomplete annulus as shown. Thus, the head can easily beremoved for cleaning by unclipping the two halves of the cowl, openingit out and sliding the cowl away from the duct, and sliding the chamberand sprays away from the duct.

A second form of apparatus according to the invention is shown in thevertical cross section in FIG. 4. This second apparatus differs from thefirst described apparatus firstly in the design of the cowl 9, whichencloses the air passage 20 and conveyer feed 12.

The cowl is provided with an aperture 11 to permit the insertion of aconveyor 12 that is typically a screw feed conveyor 13 and that leadsfrom a hopper or other suitable store 14.

An air inlet 15 is provided in the top of the cowl and a rotatablepartial closure 16 is mounted, on means not shown, above the opening 15so that the area of the opening can be adjusted by rotation of thepartial closure 16. Thus the air stream through the air passage 20 iscontrolled by valve 16.

If desired, this second form of apparatus may be provided with holes 24.It may be provided with non-stick material on the interior surfaces ofthe duct 1, duct inlet 4, jets 19 and side feed T-piece 21 and with heattape on the external surface of the side feed T-piece 21.

In this second apparatus the air passage 20 terminates at a passageoutlet 22 slightly above the weir 6. The passage 20 has substantiallythe same diameter as the duct inlet 4. With this system also very littlepowder falls into the annulus 23 around the chamber 7 since the airpassage 20 discharges directly into the duct inlet 4.

In a typical process using either of the particular forms of apparatusdescribed above, the duct 1 has a radius of about 12 cm and a lengthfrom the top of the duct to the sprays of about 8 cm and from the spraysto the bottom of the duct of about 24 cm. The cowl 9 has a depth ofabout 5 cm. The diameter of the duct inlet 4 is about 8 cm, and thediameter of the passage 20 is slightly smaller. Eight spray jets 19(shown as 19A, 19B and 19C in FIG. 1) are arranged at an angle of 10° tothe vertical close to the outer diameter of the duct. Four (19A) have acone angle of 145° while two (19B) have a cone angle of 60° and two(19C) have a cone angle of 90°.

Water soluble cationic polyelectrolyte powder is fed by the screw feed13 at a rate of 1-3 Kg per minute while water is pumped through thepipes 8 at a rate of about 1 litre per minute and the pipes 18 at a rateof 100-200 litres per minute under a pressure of 1.38×10⁵ to 4.14×10⁵ Pa(20-60 psi).

If the second apparatus described above is used, the process isinitiated with the aperture 15 fully open but once stabilised conditionshave been achieved the closure 16 is adjusted to reduce the air flowthrough the air inlet 15 to a value at which substantially no fines areblown out of the bottom of the duct but at which uniform wetting of theparticles is still achieved. If the first apparatus is used the valve 16is found to be unnecessary.

A vessel 30 for containing water (not shown) in which the polymericmaterial can be dissolved or dispersed is positioned beneath the openend of the duct. This vessel may be a tank or a channel through whichthe dissolution water is continuously flowing.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:

We claim:
 1. Apparatus for uniformly wetting water-soluble orwater-swellable particulate material comprising:a substantially verticalwetting duct defined by a wall and which has an upper part and a lowerend and which is open at its lower end, a duct inlet which is positionedin the upper part of the duct and substantially coaxial with the ductand which has a radius less than the radius of the duct and which isdefined by inlet walls which have an upper edge and which extenddownwards from the edge, feed means for feeding particulate material tothe duct inlet, water spray orifices positioned within the upper part ofthe duct radially outwards of the duct inlet and arranged to directsprays of water downwardly in the wetting duct to wet the particulatematerial, and means for providing a substantially continuous flow ofwater over the upper edge of the inlet walls and down alongsubstantially the entire exposed inner surface of the inlet walls. 2.Apparatus according to claim 1 in which the means for providing asubstantially continuous flow of water comprise an annular vesselsurrounding the duct inlet and ducting for continuously supplying waterto the annular vessel.
 3. Apparatus according to claim 1 for uniformlywetting water soluble or water swellable particulate material which isadditionally friable in which the water spray orifices are arranged todirect sprays of water downwardly and substantially across the axis ofthe duct to disintegrate into substantially independent particlesfriable particulate material falling into the duct from the duct inletand to wet the substantially independent particles.
 4. Apparatusaccording to claim 3 including as the feed means conveyer feed means forcarrying friable material to a position above the duct inlet and fordropping the material into the duct inlet.
 5. Apparatus according toclaim 4 including a hopper and wherein the conveyer feed means arepositioned to carry friable material from the hopper to a position abovethe duct inlet and for dropping the material into the duct inlet. 6.Apparatus according to claim 3 including an air feed passage that leadsfrom an air inlet that is exposed to the atmosphere, past a side feedposition for receiving a feed of friable material, to a passage outletthat discharges into the duct inlet.
 7. Apparatus according to claim 6in which the passage outlet is of smaller diameter than and ispositioned inside the duct inlet.
 8. Apparatus according to claim 6including a hopper for storing particulate material and a conveyer screwfeed that leads from the hopper into the side feed position. 9.Apparatus according to claim 1 in which the sprays of water are arrangedto provide a substantially uniform distribution of water dropletsthroughout the wetting duct.
 10. Apparatus according to claim 1comprising a cowl that substantially entirely encloses the top of theduct inlet and that includes air inlet means and an aperture in the cowlto permit the feeding of particulate material to the duct inlet fromoutside the cowl.
 11. Apparatus according to claim 10 in which the airinlet means are substantially coaxial with the duct.
 12. Apparatusaccording to claim 1 in which some of the sprays have a cone anglesubstantially greater than others of the sprays and the sprays withdifferent cone angles are substantially uniformly distributed around theupper part of the duct.
 13. Apparatus according to claim 1 includingholes provided in the wall of the wetting duct in its upper part, inwhich the holes are substantially equally spaced around the wetting ductwall and in which each hole is positioned between two adjacent waterspray orifices.
 14. Apparatus according to claim 1 additionallycomprising a vessel that can contain water and which is positionedbeneath the lower end of the duct for receiving wetted particulatematerial from the duct.
 15. A method of uniformly wetting water-solubleor water swellable particulate material comprising:providing anapparatus comprising;a substantially vertical wetting duct which has anupper part and a lower end and which is open at its lower end, a ductinlet which is positioned in the upper part of the duct andsubstantially coaxial with the duct and which has a radius less than theradius of the duct and which is defined by inlet walls which have anupper edge and which extend downwards from the edge, feed means forfeeding particulate material to the duct inlet, water spray orificespositioned within the upper part of the duct radially outwards of theduct inlet and arranged to direct sprays of water downwardly in thewetting duct to wet particulate material falling through the duct, meansfor providing a substantially continuous flow of water over the upperedge of the inlet walls and down along substantially the entire exposedinner surface of the inlet walls, feeding the particulate material tothe duct inlet while causing water to flow continuously over the upperedge of the duct inlet walls and down along substantially the entiresurface of the duct inlet walls and while directing water sprays fromthe water spray orifices and thereby wetting the material, andcollecting the wetted material in water in a vessel positioned beneaththe lower end of the duct.
 16. A method according to claim 15 ofuniformly wetting water soluble or water swellable particulate materialwhich is additionally friable additionally comprising directing watersprays from the water spray orifices so as to disintegrate the friablematerial into substantially independent particles and wet the particles.17. A method according to claim 16 comprising directing water spraysfrom the water spray orifices so as to provide a substantially uniformdistribution of water droplets throughout the wetting duct.
 18. A methodaccording to claim 15 comprising directing water sprays from the waterspray orifices so as to provide a substantially uniform distribution ofwater droplets throughout the wetting duct.